Our invention relates to solid dielectric capacitors and more particularly to ceramic capacitors of the monolithic type which are capable of manufacture by cosintering of the ceramic body and the electrodes at such low temperatures as to permit use of a base metal as the electrode material. Our invention also specifically pertains to a process for the fabrication of such ceramic capacitors.
Multilayered ceramic capacitors have long been known and used extensively which employ noble metals such as platinum and palladium as the electrode materials. Generally, for the manufacture of such capacitors, there are first prepared "green" (unsintered) dielectric sheets from the proportioned ingredients of a desired dielectric ceramic material in finely divided form. An electroconductive paste containing powdered platinum or palladium is then "printed" on the green sheets in a desired pattern. A plurality of such printed green sheets are stacked up, pressed together, and sintered in a temperature range of 1300.degree. to 1600.degree. C. in an oxidative atmosphere.
This conventional method makes possible the simultaneous firing (cosintering) of the dielectric ceramic layers and the film electrodes interleaved therewith. It is also an acknowledged advantage of the known method that the noble metal electrodes are totally unaffected by the high temperature sintering in an oxidative atmosphere. Offsetting all these advantages is the expensiveness of the noble metals, which add considerably to the costs of the multilayered ceramic capacitors.
Japanese Laid Open Patent Application No. 53-98099 suggests a solution to the above discussed problem, teaching ceramic compositions consisting primarily of calcium zirconate (CaZrO.sub.3) and manganese dioxide (MnO.sub.2). In the manufacture of ceramic capacitors the dielectric bodies of these known compositions are sinterable in a reductive atmosphere, so that electrodes of nickel or like base metal can be employed for cosintering with the dielectric bodies without the danger of oxidation.
We do, however, object to the prior art CaZrO.sub.3 -MnO.sub.2 ceramic compositions for several reasons. These known ceramic compositions require firing in as high a temperature range as from 1350.degree. to 1380.degree. C. When the green sheets of the ceramic compositions, having printed thereon a paste composed primarily of powdered nickel, are sintered in that temperature range, the nickel particles tend to grow and flocculate in spite of the nonoxidative atmosphere in which they are fired. We have also found that the base metal particles are easy to diffuse into the ceramic bodies when fired in that temperature range. The flocculation and diffusion of the base metal particles are, of course, both undesirable as the resulting capacitors will in all likelihood fail to possess desired values of capacitance and insulation resistance.
These weaknesses of the CaZrO.sub.3 -MnO.sub.2 ceramic compositions have been overcome to some extent by Kishi et al. U.S. Pat. Nos. 4,700,265 and 4,709,299, both teaching dielectric ceramic compositions that permit cosintering at temperatures of not more than 1200.degree. C. The compositions according to U.S. Pat. No. 4,700,265 consist essentially of a major ingredient expressed by the general formula, (SrO).sub.k (Zr.sub.1-x Ti.sub.2)O.sub.2, and minor proportions of lithium oxide (Li.sub.2 O), silicon dioxide (SiO.sub.2), and at least one metal oxide selected from among barium oxide (BaO), magnesium oxide (MgO), zinc oxide (ZnO), strontium oxide (SrO) and calcium oxide (CaO). U.S. Pat. No. 4,709,299, on the other hand, proposes compositions consisting essentially of a major ingredient expressed by the general formula, (CaO).sub.k (Zr.sub.1-x Ti.sub.x)O.sub.2, and minor proportions of boric oxide (B.sub.2 O.sub.3), SiO.sub.2, and at least one metal oxide selected from among BaO, MgO, ZnO, SrO and CaO. compositions according to U.S. Pat. No. 4,700,265.
The ceramic compositions suggested by the noted U.S. patents permit cosintering of the dielectric bodies and base metal electrodes in a reductive or neutral atmosphere at temperatures not exceeding 1200.degree. C. Little or no flocculation of the base metal, particularly nickel, takes place because of the low sintering temperatures. The resulting ceramic capacitors are particularly well suited for temperature compensating applications, having a specific dielectric constant of not less than 30, a temperature coefficient of capacitance of -800 to +140 parts per million (ppm) per degree centigrade (C.), a Q factor of not less than 2000, a resistivity of not less than 1.times.10.sup.7 megohm-centimeters (megohm-cm), and a bending strength of approximately 1300 kilograms per square centimeter (kg/cm.sup.2).
While these performance characteristics of the closest prior art are satisfactory for all practical purposes, we have nevetheless been hard pressed by our customers for dielectric ceramics of higher performance characteristics. For example, an improvement in resistivity will result in capacitors of greater voltage withstanding capability. An improvement in bending strength will result in less breakage of capacitors during their manufacture and mechanical mounting on circuit boards. An improvement in specific dielectric constant will result in capacitors of higher capacitance or, if the capacitance is maintained the same as heretofore, in capacitors of greater interelectrode spacing. The greater interelectrode spacing is desirable from the standpoint of less voltage per unit thickness of the ceramic body.